Anons. This is more serious than just one agency. MI6 and the CIA differ in one fundamental way. The CIA is independent from the government. It is a civilian Intelligence agency. There is a firewall between the agency and government. This is not the case with MI6, which reports directly to the Foreign Office (The UK version of the State Department).
https://en.wikipedia.org/wiki/Chief_of_the_Secret_Intelligence_Service
If MI6 did these reprehensible things then the chain of command is quite clear and leads up the chain thus -
Individual in MI6
Team leader/Managers within MI6
Head of MI6
Head of Foreign Office
Prime Minister
In practice there is also something called the Joint Intelligence Committee where various branches of the intelligence community meet
https://en.wikipedia.org/wiki/Chief_of_the_Secret_Intelligence_Service